The invention relates generally to electronic stethoscopes, and more particularly to electronic stethoscopes employing active noise reduction circuitry to permit detection of auscultory sounds in patients in high ambient noise environments such as aircraft or moving ambulances.
The Gosport tube is presently the most common type of stethoscope, employing a diaphragm for conduction of sound through rubber tubing into binaural earplugs. Electronic stethoscopes are available which use in-line electronic amplifiers to boost low-frequency auscultory sounds that typically lie in the frequency range between 10 Hz and 250 Hz. Regardless of amplification, the Gosport tube approach to auscultation fails in areas of high ambient noise. Trauma rooms, ambulances, and aircraft are examples of areas plagued by low frequency background sounds. In the case of helicopter operations these sounds may reach amplitudes of 120 dB. Regardless of the degree of amplification of heart and lung sounds, the signal-to-noise ratio remains high and usually preclude useful listening.
Current ambient noise reduction techniques have proved to be of only marginal value to stethoscopic applications for aircraft and the like. It is known that ambient noise penetrating the transducer wall can be reduced by low-pass filtering the transducer output, or by the use of heavy acoustical shielding around the body of the transducer to enhance the ausculation sounds emanating from the body wall of the patient. An electronic stethoscope employing low-pass filtering combined with passive shielding of the pickup transducer is disclosed in Brogan, F. A., Collins, F. G., and Wing, M. E., An Experimental Electronic Stethoscope for Aircraft Use, USAF Rep. No. SAM-TR-67-39 (1966). These techniques, however, are ineffective to reduce ambient noise impinging directly on the ear.
One approach to reducing ambient noise at the ear of the listener is to employ a negative feedback loop from a summing microphone located near the ear canal to the speaker generating the desired audio signal, in effect broadcasting "anti-noise" to cancel the ambient noise. It is known to use negative feedback of a noisy audio signal to reduce ambient noise ("active noise reduction") in a stethoscopic application. U.S. Pat. No. 4,985,925 issued to Langberg et al. discloses active noise reduction circuitry for a stethoscope having earplugs. Such a stethoscope, however, still has the disadvantage that, in extremely high ambient noise environments, the ambient noise impinging on the summing microphone is of such a magnitude that the speaker cannot generate a sufficiently strong "anti-noise" signal to cancel the noise signal.
Moreover, a stethoscope user in evacuation and transportation vehicles often needs to be in communication with others in the vehicle but has difficulty alternating between his stethoscope and the vehicle's intercom system. This situation commonly occurs in combat aircraft, where the medical personnel may also be part of the flight crew. A disadvantage of the prior art stethoscopes is that they are not integrable into the intercom system of an evacuation or transportation vehicle.
Accordingly, an object of the present invention is to provide an improved stethoscopic system for reducing high ambient noise in environments such as aircraft, ambulances, and the like.
Another object of the present invention is to provide a stethoscopic system that is easily integrable into a vehicular intercom system to permit medical personnel to contemporaneously monitor the patient and communicate with other personnel in the vehicle.
Other objects of the present invention will become apparent from the specification.